(1) Field of the Invention
The present invention relates to a mobile terminal apparatus and a channel compensation method of the mobile terminal apparatus. The present invention relates to, for example, a preferable art employed in an apparatus for communicating by an HSDPA (High Speed Downlink Packet Access) transmission system, which is one of radio mobile terminal transmission systems and having a function to temporarily store information to be discarded and decode by using both of stored information and retransmitted information.
(2) Description of Related Art
An HSDPA for providing the maximum transmission rate of 14 Mbps in downlink communications from a base station to a mobile terminal is currently specified in the 3GPP (the 3rd Generation Partnership Project) as a theme of standardization of W-CDMA (Wideband-Code Division Multiple Access) system, which is one of the third generation mobile communication systems.
The HSDPA transmission system is an art for changing the number of multicodes, a modulating system (such as QPSK or 16QAM), a transmission block size (TBS: Transport Block Size) or the like of an HS-PDSCH (High Speed-Physical Downlink Shared Channel), which will be described later, according to a reception environment of a mobile terminal in order to select the most appropriate transmission rate and perform communications.
The HSDPA employs an adaptive coding modulation system and, for example, it is characterized by adaptively switching the QPSK modulation system and 16QAM system according to a radio environment between a base station and a mobile terminal.
The HSDPA system employs a retransmission control system called HARQ (Hybrid Automatic Repeat reQuest). In the HARQ system, when a mobile terminal detects an error in data received from the base station, the mobile terminal requests the base station to retransmit the data. Accordingly, the data is retransmitted from the base station. The mobile terminal carries out an error correction decoding in use of both the previously received data and the retransmitted reception data. In the HARQ system, effective utilization of the previous reception data increases a gain of the error correction decoding and reduces the number of retransmission.
As major radio channels employed in the HSDPA, there are HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Both of the HS-SCCH and the HS-PDSCH are downlink shared channels and the HS-SCCH is a control channel for transmitting various parameters related to data transmitted by the HS-PDSCH. The parameters includes, for example, modulating type information indicating which modulation system is employed to transmit data by the HS-PDSCH, the number of allocation of spread coding (the number of codes), or patterns of rate matching processes performed on transmission data.
On the other hand, the HS-DPCCH is an uplink dedicated control channel for communication from the mobile terminal to the base station. The HS-DPCCH is used when the mobile terminal transmits ACK signal and NACK signal to the base station according to a result of data reception of HS-PDSCH. When the mobile terminal fails to receive data (for example, CRC error in the received data), NACK signal is transmitted from the mobile terminal, so that the base station carries out retransmission control. When the CRC error occurs in HS-SCCH, HS-PDSCH cannot be correctly decoded and DTX signal, which indicates that HS-PDSCH addressed to the mobile terminal itself is not received, is transmitted to the base station. In this case, the base station retransmits the same data.
Further, the HS-DPCCH is used also when the mobile terminal measures reception quality (for example, SIR: Signal Interference Ratio) of the signal received from the base station and periodically transmits the result to the base station as CQI (see FIG. 4). The base station determines a condition of the downlink radio environment based on the received CQI. When the environment is good, the base station may switch to a modulation system for higher data transmission rate and when the environment is not good, the base station adaptively switches to a modulation system for lower data transmission rate.
(Channel Structure)
Next, a channel structure of the HSDPA will be described.
FIG. 4 is a diagram showing a channel structure of HSDPA. It is noted that, in the W-CDMA system, each channel is separated by coding to be adapted to a code division multiplex system.
Firstly, channels, which are yet to be described among the channels shown in FIG. 4, will be described.
A CPICH (Common Pilot Channel) and a P-CCPCH (Primary Common Control Physical Channel) are respectively downlink shared channels. The CPICH is a channel used for channel estimation, cell search, and a timing basis of other downlink physical channels in the same cell and used to transmit so-called pilot signals (known signals between the base station and the mobile terminal). The P-CCPCH is a channel for transmitting broadcasting information.
Next, timing relationship in each channel will be described.
As shown in FIG. 4, each channel includes a frame (10 ms) that is composed of 15 slots. As described above, the CPICH is used as a basis of other channels and the beginning of frame of the P-CCPCH is respectively corresponding to the beginning of frame in the HS-SCCH. Here, the beginning of frame of the HS-PDSCH is delayed by 2 slots with respect to that of HS-SCCH. This delay is provided in order to notify, in advance, modulating type information or spread code information, which are required for demodulating the HS-PDSCH in the mobile terminal.
Accordingly, the mobile terminal performs HS-PDSCH demodulation or the like by selecting the corresponding demodulating system and despreading code according to the notified information via the HS-SCCH. Further, the HS-SCCH and the HS-PDSCH include a sub-frame composed of 3 slots. The foregoing is the brief descriptions of the HSDPA channel structure.
(Structure of Mobile Terminal)
FIG. 5 is a diagram showing a structure of a relevant part of a known mobile terminal adapted to the HSDPA. The mobile terminal shown in FIG. 5 includes, for example, a receiver 101, an HS-SCCH channel estimation filter 102, an HS-SCCH channel compensator 103, an HS-SCCH demodulator 104, an HS-SCCH decoder 105, an HS-SCCH-CRC calculator 106, an HS-PDSCH symbol buffer 107, an HS-PDSCH channel estimation filter 108, an HS-PDSCH channel compensator 109, an HS-PDSCH demodulator 110, an HS-PDSCH decoder 111, a retransmitting (HARQ) processor 112, a retransmitting (HARQ) buffer 113, an HS-PDSCH-CRC calculator 114, a downlink reception timing monitor 115, an uplink transmission timing manager 116, a scheduler 117, an encoder 118, a modulator 119, and a transmitter 120.
In the mobile terminal, a reception signal received by a reception antenna (not shown) is input into the receiver 101. The receiver 101 performs processes such as path detection or despreading for downlink and separates each channel of CPICH, HS-SCCH, and HS-PDSCH. The separated CPICH is input into the HS-SCCH channel estimation filter 102 and the HS-PDSCH cannel estimation filter 108, respectively.
The HS-SCCH channel estimation filter 102 and the HS-PDSCH channel estimation filter 108 calculate channel estimation values of HS-SCCH and HS-PDSCH respectively according to the pilot signals received via CPICH.
On the HS-SCCH which is separated in the receiver 101, the HS-SCCH channel compensator 103 carries out channel compensation for HS-SCCH in use of a channel estimation value obtained in the HS-SCCH channel estimation filter 102, the HS-SCCH demodulator 104 carries out demodulation, the HS-SCCH decoder 105 carries out decoding and the HS-SCCH-CRC calculator 106 carries out CRC calculation. Since the information whose CRC is determined as OK in the HS-SCCH-CRC calculator 106 includes, as described above, information required for HS-PDSCH decoding such as modulating type information and spread code information, it is provided to the HS-PDSCH decoder 111. Here, when the result of HS-SCCH-CRC calculation is NG, an error (DTX) is notified to the scheduler 117.
On the other hand, the HS-PDSCH which is separated in the receiver 101 is firstly buffered and delayed in the HS-PDSCH symbol buffer 107. Then, the HS-PDSCH channel compensator 109 carries out channel compensation in use of the channel estimation value obtained in the HS-PDSCH channel estimation filter 108 and the HS-PDSCH demodulator 110 carries out demodulation. Further, the HS-PDSCH decoder 111 decodes in use of necessary information for HS-PDSCH decoding obtained in the HS-SCCH-CRC calculator 106.
Here, as described later with reference to FIG. 7, the HS-PDSCH symbol is delayed in the HS-PDSCH symbol buffer 107 because it is preferable to use a channel estimation value that is calculated by averaging CPICH symbols of a plurality of past and future slots with respect to a target HS-PDSCH slot (symbol) of the demodulation.
The HS-PDSCH symbol which is decoded by the HS-PDSCH decoder 111 is input into the HARQ processor 112 and accordingly stored in the HARQ buffer 113 in preparation for retransmission combining process based on HARQ. Accordingly, when retransmitting, the HARQ processor 112 combines the previously received HS-PDSCH symbol stored in the HARQ buffer 113 and the retransmitted HS-PDSCH symbol to input into the HS-PDSCH-CRC calculator 114.
The HS-PDSCH-CRC calculator 114 carries out CRC calculation on the HS-PDSCH symbol from the HARQ processor 112. Then, when the result is OK, ACK signal is transmitted to the scheduler 117 and when the result is NG, NACK signal is transmitted to the scheduler 117, respectively as calculation results.
The scheduler 117 schedules DTX from the HS-SCCH-CRC calculator 106, and ACK/NACK from the HS-PDSCH-CRC calculator 114, respectively, in accordance with the transmission timing signal from the uplink transmission timing manager 116. That is, as shown in the last line in FIG. 4, the scheduler 117 schedules so that ACK/NACK(/DTX) are respectively transmitted 7.5 slots later from the reception of HS-PDSCH. Here, the reception of HS-PDSCH is monitored by the downlink reception timing monitor 115.
On each information scheduled as described above, the encoder 118 encodes as HS-DPCCH data and the modulator 119 modulates. Then, the transmitter 120 transmits that information to the base station via HS-DPCCH. The base station transmits new data when receiving ACK, retransmits HS-SCCH and HS-PDSCH when receiving DTX, and retransmits HS-PDSCH when receiving NACK.
(HARQ Processing Flow)
Next, an HARQ processing flow according to the known art will be described below with reference to FIG. 6.
As described above, after the HS-SCCH decoding, the mobile terminal transmits DTX to the base station when the result of CRC calculation of HS-SCCH is NG (from NG route in step S101 to step S110) and determines whether or not it is new data or retransmission data when the result of CRC calculation of HS-SCCH is OK (from OK route in step S101 to step S102).
When it is determined as new data, HS-PDSCH CRC calculation is carried out after HS-PDSCH decoding (from ‘new data’ route in step S102 to step S103, and step S104). When the result of the CRC calculation is OK, ACK is transmitted to the base station (from OK route in step S103 to step S108). When the result of CRC calculation is NG, NACK is transmitted to the base station (from NG route in step S104 to step S109) and HARQ information is stored in the HARQ buffer 113.
On the other hand, when it is determined as retransmission data in the step S102, the HARQ processor 112 combines the received data and previous addition data stored in the HARQ buffer 113 (from ‘retransmission data’ route in step S102 to step S105). After the HS-PDSCH decoding, HS-PDSCH CRC calculation is carried out (steps S106 and S107). ACK is transmitted to the base station when the CRC calculation result is OK (from OK route in step S107 to step S108) and NACK is transmitted again to the base station when the CRC calculation result is NG (from NG route in step S107 to step S109).
In this way, the mobile terminal carries out error correction decoding by using both of the previous reception data and the retransmitted reception data, so that a gain of the error correction decoding is increased and the number of retransmission is reduced.
There is an art proposed by Japanese Patent Application Laid-Open No. 2004-248196, which is related to the HSDPA system. Japanese Patent Application Laid-Open No. 2004-248196 discloses an art for selecting a high quality bit having good reception quality and combining the bit with a retransmission signal in order to improve reception characteristics of a data channel.
FIG. 7 is a diagram showing a time chart image of HS-PDSCH channel estimation and compensation performed in the mobile terminal shown in FIG. 5 by symbol unit. FIG. 8 is a diagram showing a time chart image of HS-SCCH channel estimation and compensation performed in the mobile terminal shown in FIG. 5 by symbol unit. In FIGS. 7 and 8, the number of symbols in a single slot is defined as 10, from #0 to #9.
According to the HS-PDSCH modulation in the mobile terminal, in order to modulate a slot (for example, slot #n in FIG. 7), a channel estimation value which is appropriate to the time of slot #n is required to be calculated from CPICH symbol in order to carry out the modulation process on HS-PDSCH symbol. Accordingly, the channel estimation value which is appropriate to the time of slot #n (filtering process) is preferably calculated by averaging (each “Σ” in FIGS. 7 and 8 represents an averaging process) past and future CPICH symbols (slot #n−1 to slot #n+1), however, in this case, it gets to the time of slot #n+1 before the channel estimation process is completed.
Therefore, in the mobile terminal, as indicated by an arrow 200 in FIG. 7, the HS-PDSCH symbol buffer 107 delays HS-PDSCH symbol of slot #n and the modulation process is carried out at the time from slot #n+1 to slot #n+2.
Here, it is specified that, in the HSDPA, as described above, ACK/NACK signal is transmitted to the base station at 7.5 slots later from the completion of HS-PDSCH reception and HS-PDSCH is received at 2 slots later from the reception of HS-SCCH. In order to complete the HS-PDSCH decode process on a data signal transmitted by about 14 Mbps, which is the maximum throughput in the HSDPA, before the ACK/NACK(/DTX) transmission, information required for HS-PDSCH decoding (HS-PDSCH decode information) needs to be obtained by carrying out the HS-SCCH demodulation and decode processes within one slot.
Therefore, in order to demodulate a slot of HS-SCCH (for example, slot #n in FIG. 8), for example, it is preferable to carry out a modulation process by using a channel estimation value calculated from past and future CPICH symbols with respect to the reception symbol (for example, CPICH symbols from slot #n−1 to slot #n+1 in FIG. 8) while delaying an HS-SCCH reception signal by one slot (buffering process) similar to the demodulation process of HS-PDSCH. However, because of the above temporal restriction, the HS-SCCH reception symbol cannot be delayed (buffered).
Therefore, for demodulation of HS-SCCH on slot #n, a future CPICH symbol cannot be used and a demodulation process is carried out in use of a channel estimation value calculated from only past CPICH symbols (for example, CPICH symbols from slot #n−2 to slot #n).
In other words, in FIG. 7, focusing attention on symbol #0 of slot #n, channel estimation value filtering for symbol #0 of HS-PDSCH is carried out in use of CPICH symbols from the first CPICH symbol in the past slot #n−1 to the last CPICH symbol in the future slot #n. Accordingly, a channel estimation value can be calculated from past and future CPICH symbols (slot #n−1 to slot #n+1) with respect to the time of symbol #0 of HS-PDSCH.
As shown in FIG. 8, since channel estimation value filtering for symbol #0 in slot #n of HS-SCCH is needed to be carried out by the last CPICH symbol in past slot #n−1, the filtering (averaging process) is carried out in use of CPICH symbols from the first CPICH symbol in slot #n−2 to the last CPICH symbol in slot #n−1.
Therefore, since a channel estimation value with respect to the time of the first symbol in slot #n−1 of HS-SCCH (one slot prior to the time of symbol #0) is calculated, a channel estimation value which is not appropriate to the time of symbol #0 may be calculated in some reception environments. As a result, the reception quality of HS-SCCH is often lower than the reception quality of HS-PDSCH under an environment in which a channel estimation result may change within a short time because of high speed fading or the like and a past channel estimation value differs from a current channel estimation value.
As described above, in general, a channel having higher error tolerance is allocated to HS-SCCH than HS-PDSCH so that the reception quality is usually better in HS-SCCH than in HS-PDSCH. However, because of the temporal restriction in the demodulation process, only past CPICH symbols may be used for the channel estimation for HS-SCCH modulation. As a result, the relation of reception qualities of HS-SCCH and HA-PDSCH is reversed in some radio environments with high speed fading.
Such phenomenon will be explained with reference to FIGS. 9 and 10. FIG. 9 is a graph quantitatively showing HS-PDSCH BLER (Block Error Rate) characteristics when receiving a fixed format corresponding to a fading speed. FIG. 10 is a graph quantitatively showing HS-SCCH BLER characteristics corresponding to a fading speed.
As shown in FIG. 9, BLER of HS-PDSCH is substantially constant with respect to a fading speed; however, as shown in FIG. 10, BLER of HS-SCCH is deteriorated as the fading speed increases. In this manner, the reception quality of HS-SCCH tends to be deteriorated comparing to the reception quality of HS-PDSCH under an environment, such as fading environment, in which temporal phase changes are quantitatively generated.
Therefore, in case of high speed fading, even when the reception quality of HS-PDSCH is comparatively good, the reception quality of HS-SCCH is deteriorated. Accordingly, the CRC calculation result of HS-SCCH is determined as NG, and a decode process of HS-PDSCH cannot be performed. Here, since DTX is transmitted to the base station, combining of reception waves is not performed in the HARQ process. As a result, a reception speed is reduced and throughput in the mobile terminal and throughput in the system may be decreased. This phenomenon occurs also in the art disclosed in Japanese Patent Application Laid-Open No. 2004-248196.